Currently known printing methods adopted by printing apparatuses include: a thermal transfer method which utilizes thermal energy to transfer ink of an ink ribbon to a printing medium such as printing paper; an inkjet printing method which performs printing by discharging an ink droplet to be attached to a printing medium such as printing paper; and so forth.
In particular, a printing apparatus employing an inkjet printing method is widely adopted by printers, copying machines or the like, since it has advantages of low noise, low running cost, ease of downsizing the apparatus, and ease of color printing.
The aforementioned printing apparatus employing an inkjet printing method generally utilizes a printhead which has a plurality of integrally arranged printing elements for improved printing speed. The printing elements normally include nozzles and ink discharge orifices for discharging ink. The major problem of such inkjet printing apparatus is printing unevenness. Differences in ink discharging characteristics of the plural nozzles cause density unevenness in a printed image.
Various methods have been proposed to prevent such unevenness and achieve high quality. For instance, according to a multi-pass printing method, one printing region of a printing medium is printed by scanning a printhead plural numbers of times. To make full use of the multi-pass printing, the number of times of scanning in one printing region must be increased, in other words, the number of division must be increased. This causes a reduced throughput.
As another method of preventing an occurrence of density unevenness without using the divisional printing method, Japanese Patent Application Laid-Open No. 5-69545 proposes a head shading method.
According to this method, a predetermined test pattern for determining a correction value is first printed by a printhead on a printing medium. The image of the printed test pattern is read by a scanner having a CCD (Charged Coupled Device) or the like, then appropriate position correction is performed on the read image, and image density thereof is allocated to respective rasters corresponding to nozzles of the printhead. A variation of the printing density is caused by errors in the amount of discharge from each nozzle, deviation of the ink discharge direction, ink blur on a printing medium, and the like.
Based on the density allocated for each raster, a correction value of the printing density for each nozzle is determined. Based on the correction value, a γ table for each nozzle or a driving table for each nozzle are changed to adjust the amount of ink discharge. For a raster whose printing density is high in the non-correction state, density correction such as output γ correction is performed so as to decrease the density. For a raster whose printing density is low in the non-correction state, density correction such as output γ correction is performed so as to increase the density. The density unevenness is reduced by the above-described correction.
However, the conventional head shading method has a problem in that it takes a long time to measure the printing density of the test pattern.
Particularly, since the number of nozzles of a recent inkjet printhead extends to several thousands, the region subjected to correction is expanding. Furthermore, in order to meet the recent demands for high quality images, high precision is required to measure the printing density of the test pattern.
Since it is necessary to perform density measurement in a wide region with high precision, the conventional density correction method is approaching its limit in the effort to reduce the measurement time.